The subject matter herein relates generally to cyclotrons and, more specifically, to mechanisms for reducing irradiation of surfaces or components within the cyclotrons.
A cyclotron is a type of particle accelerator in which a beam of charged particles (e.g., H− charged particles or D− charged particles) are accelerated outwardly along a spiral orbit. Cyclotrons may be used to generate radioisotopes (also called radionuclides), which have several applications in medical therapy, imaging, and research, as well as other applications that are not medically related. In such systems, the cyclotron directs the beam into a target material to generate the isotopes.
The cyclotron includes an ion source that provides the charged particles into an acceleration chamber of the cyclotron. The cyclotron uses electrical and magnetic fields to accelerate and guide the charged particles along a predetermined orbit within the acceleration chamber. The magnetic fields are provided by electromagnets and a magnet yoke that surrounds the acceleration chamber. The electrical fields are generated by a pair of radio frequency (RF) electrodes (or dees) that are located within the acceleration chamber. The RF electrodes are electrically coupled to an RF power generator that energizes the RF electrodes to provide the electrical field. The electrical and magnetic fields cause the charged particles to take a spiral-like orbit that has an increasing radius. When the charged particles reach an outer portion of the orbit, the charged particles are stripped of their electrons and form a particle beam that is directed toward the target material for isotope production.
As the charged particles are guided along the orbit, however, the charged particles may collide with other particles, such as residual gas molecules from the ion source or other gas molecules generated by outgassing, degassing, or desorption within the acceleration chamber. An ion may become a neutral particle upon colliding with the other particle. The neutral particle has a trajectory that is essentially tangent to the point in the orbit at which the ion collided with the other particle. The neutral particle then collides with other surfaces in the acceleration chamber, such as the RF electrodes or the extraction system. In the case of RF electrodes, these parts often comprise copper (or other conductive material). When a proton or a neutral hydrogen collides with copper, a relatively large amount of gamma and neutron radiation is generated and long-lived isotopes (e.g., Zn-65) may be generated. This is often the primary source of radiation within an acceleration chamber. Due to the geometry of the cyclotron in the acceleration chamber, the RF electrodes are particularly exposed to the neutral particles.
When service personnel open the acceleration chamber, the personnel are exposed to the activated parts. As such, the accumulation of induced by-products from radiation may be a hazard to individuals. Moreover, an excessive amount of radiation may make it necessary to replace a part earlier than expected.